The present invention is generally directed to cooling systems and, more particularly, to an enhanced cooling system having a passive first stage, for reducing temperature of a cooling fluid to near ambient temperature or above, and an active second stage, for further reducing temperature of the cooling fluid below ambient temperature. The cooling system may be used for enhanced cooling of heat generating components in, for example, an electronic device such as a mainframe computer or other electronic system requiring cooling.
As is well known, as the circuit density of electronic chip devices increases in order to achieve faster and faster processing speed, there is a correspondingly increasing demand for the removal of heat generated by these devices. The increased heat demand arises both because the circuit devices are packed more closely together and because the circuits themselves are operated at increasingly higher clock frequencies. Nonetheless, it is also known that runaway thermal conditions and excessive heat generated by chips is a leading cause of failure of chip devices. Furthermore, it is anticipated that the demand for heat removal from these devices will increase indefinitely. Accordingly, it is seen that there is a large and significant need to provide useful cooling mechanisms for electronic circuit devices.
The use of large thermoelectric cooling elements is known. These elements operate electronically to produce a cooling effect. By passing a direct current through the legs of a thermoelectric device, a temperature difference is produced across the device which may be contrary to that which would be expected from Fourier""s Law.
At one junction of the thermoelectric element both holes and electrons move away, toward the other junction, as a consequence of the current flow through the junction. Holes move through the p-type material and electrons through the n-type material. To compensate for this loss of charge carriers, additional electrons are raised from the valence band to the conduction band to create new pairs of electrons and holes. Since energy is required to do this, heat is absorbed at this junction. Conversely, as an electron drops into a hole at the other junction, its surplus energy is released in the form of heat. This transfer of thermal energy from the cold junction to the hot junction is known as the Peltier effect.
Use of the Peltier effect permits the surfaces attached to a heat source to be maintained at a temperature below that of a surface attached to a heat sink. What these thermoelectric modules provide is the ability to operate the cold side below the ambient temperature of the cooling medium (air or water). When direct current is passed through these thermoelectric modules a temperature difference is produced with the result that one side is relatively cooler than the other side. These thermoelectric modules are therefore seen to possess a hot side and a cold side, and provide a mechanism for facilitating the transfer of thermal energy from the cold side of the thermoelectric module to the hot side of the module.
By varying the electrical current passing through a thermoelectric module, thermoelectric cooling elements are capable of handling varying heat loads. However, one drawback of thermoelectric cooling elements is their low coefficient of performance (COP). COP refers to the ratio of the net cooling effect to the power required by the thermoelectric cooling elements. Typically, when thermoelectric modules are employed in a cooling system, substantially all of the heat transferred from a cooling fluid is transferred through the thermoelectric modules. This total transfer of heat load requires a considerable amount of power to operate the thermoelectric cooling elements effectively. Since a high amount of power is required, the COP of the entire cooling system is low.
Moreover, conventional thermoelectric modules have relatively low heat flux capability. Generally, individual thermoelectric modules exhibit relatively low heat flux capability in the order of a few watts/cm2 at any useful temperature difference from hot side to cold side.
Therefore, there is a need to further enhance cooling systems, particularly systems employing thermoelectric modules, for more efficient cooling of electronic devices or other heat generating components.
The shortcomings of the prior approaches are overcome, and additional advantages are provided, by the present invention which in one aspect is a cooling system which reduces in two or more stages the temperature of a cooling fluid exposed to a heat generating component of an electronic device. A first stage uses a first heat exchanger to reduce the temperature of the cooling fluid to near ambient temperature or above, while one or more second stages comprise a second heat exchanger employing, for example, thermoelectric modules to reduce the temperature of the cooling fluid exiting the first heat exchanger to below ambient temperature. Advantageously, heat removed from the cooling fluid by the first heat exchanger does not pass through the thermoelectric cooling elements of the second heat exchanger. Therefore, less power is required to operate the thermoelectric modules by avoiding the entire heat load being handling by the thermoelectric modules. Since less power is required, the COP of the entire system is increased.
To restate, a fluid-based cooling system for cooling a heat generating component in accordance with an aspect of the present invention employs a first stage cooling system and a second stage cooling system. The first stage cooling subsystem reduces a temperature of the cooling fluid to near ambient temperature or above. The second stage cooling subsystem is in fluid communication with the first stage cooling subsystem, and further reduces temperature of the cooling fluid exiting the first cooling subsystem to below ambient temperature.
In another aspect, the first stage cooling subsystem is a passive heat exchanger (e.g. air-cooled heat exchanger) and the second stage cooling subsystem is an active heat exchanger (e.g. employing thermoelectric modules).
In a further aspect, a method of fabricating a cooling system in accordance with an aspect of the present invention includes thermally coupling a heat transfer device to a heat generating component, the heat transfer device being configured to carry a cooling fluid; positioning a first stage cooling apparatus in fluid communication with an output of the heat transfer device, the first stage cooling apparatus being configured to reduce a temperature of the cooling fluid to near ambient temperature or above; and positioning a second heat exchanger in fluid communication with an output of the first heat exchanger, the second stage cooling apparatus being configured to reduce the temperature of the cooling fluid exiting the first stage cooling apparatus to below ambient temperature.
Advantageously, by reducing the temperature of a cooling fluid exposed to a heat generating component before passing it through a heat exchanger employing thermoelectric modules, less power is required to operate the thermoelectric modules, i.e., as long as at least a portion of the heat load is removed before being exposed to the thermoelectric modules. Since less power is required, the overall COP of the cooling system is increased.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered part of the claimed invention.